In general, present technology relating to the adhesive bonding of structural metals has the capability of providing bonded joints with satisfactory initial strength properties. The use of bonded titanium structures, however, has been limited because of the susceptibility of the interface between the titanium and the adhesive to environmental degradation. In particular, conventional titanium pre-bond surface treatments such as the phosphate-fluoride process provide an adhesive bond that although possessing adequate initial strength nevertheless lacks resistance to the degrading effects of warm, humid environments.
According to published reports, this environment disbonding problem can be mitigated through the use of an alkaline peroxide pre-bond surface treatment; alkaline peroxide treated adherends consistently have demonstrated superiority in performance over phosphate-fluoride adherends in terms of resistance to disbond in warm, humid environments. It has been speculated that the durability of the alkaline peroxide treated adherends is a result of the rough, porous and relatively stable oxide which is created on the titanium surface.
In addition to providing increased disbond resistance, the alkaline peroxide treatment offers several other advantages. For example, it utilizes a non-polluting treatment solution with relatively low toxicity and presents essentially no waste disposal problem. The treatment solution contains no fluoride which is thought to contribute to the susceptibility of the titanium surface to irreversible morphological changes in the presence of moisture. Moreover, the alkaline peroxide treatment is a one-step process and has the potential of being relatively simple and inexpensive to operate.
Implementation of the alkaline peroxide process into a commercially viable full-scale production process, however, has been hampered by the fact that the hydrogen peroxide component is inherently unstable. This instability characteristic presents quality control and solution maintenance problems uncommon to those of the conventional pre-bond surface treatment processes commonly used in production situations.
Thus, it is an overall objective of the present invention to provide an improved alkaline peroxide prebond surface treatment process that can be readily utilized to provide consistently acceptable performance in a typical factory production line environment in a cost effective manner.
To at least some extent, the decomposition of the hydrogen peroxide is caused by the presence of trace cationic impurities. Certain substances have the potential of absorbing cationic impurities and render them inactive. Avoidance of such impurities is not feasible since the titanium metal being treated contains numerous constituents which will be expected to form cationic species during the surface etching process. Accordingly, it is one specific objective to provide the process with appropriate chemical inhibitors which will react with the various cationic impurities and neutralize their effects.
From published prior art literature, it would appear that concentrations of 0.2 molar H.sub.2 O.sub.2 and 0.5 molar NaOH are the most suitable for preparing titanium for adhesive bonding; however, in practice, there will be a constant fluctuation of peroxide concentration. Moreover, the rate of decomposition of the peroxide component will be dependent upon the solution's alkalinity. Furthermore, according to at least some published reports "the [alkaline peroxide treated] surface is extremely sensitive to the peroxide bath composition," Thus, it is another specific objective to identify the concentration limits of both the hydrogen peroxide and sodium hydroxide required to obtain a high quality titanium bond. Finally, in order to adapt the process to a production environment, a simple, reliable and safe method for monitoring and maintaining the concentration within such limits is required. Inert platinum or carbon electrodes are unreliable because of contaminants in the solution; dropping mercury electrodes are potentially hazardous. Manual methods are always time consuming and prone to human error. Accordingly, a further objective is to provide a simple, reliable, timely and safe method of monitoring and maintaining the peroxide concentration within the desired limits.
Various aspects of prior art alkaline peroxide solutions and their effects on titanium are discussed in an article by A. Mahoon et al., entitled "A New Highly Durable Titanium Surface Pretreatment for Adhesive Bonding," 10th National Technical Sampe Conf., Vol. 10, Oct. 17, 1978, p. 425 et seq.; in an article by J. L. Cotter, entitled "Natural Weathering Behaviour of Adhesive Bonded Titanium Alloy Assemblies", Society of Environmental Engineering Symposium, Apr. 27, 1977; and in a report by M. Natan et al., entitled "Bondability of Ti Adherends II. Humid Environmental Effects," Martin Marietta Final Report MML TR81-42(c), Sept., 1981, which are hereby incorporated in their entirety by reference, as is an article by W. Nicoll et al., entitled "Stability of Dilute Alkaline Solutions of Hydrogen Peroxide", Industrial and Engineering Chemistry, Dec. 1955, p. 2548 et seq.